Surface acoustic wave device

ABSTRACT

A surface acoustic wave device has a high electromechanical coefficient and reflection coefficient, and also has an improved frequency-temperature characteristic that is achieved by forming a SiO 2  film on an IDT so as to prevent cracking from occurring on a surface of the SiO 2  film so that desired properties can be reliably obtained. The surface acoustic wave device includes at least one IDT, which is composed of a metal or an alloy having a density higher than that of Al and is formed on a 25° to 55° rotation-Y plate X propagation LiTaO 3  substrate, and a SiO 2  film disposed on the LiTaO 3  substrate so as to cover the at least one IDT for improving the frequency-temperature characteristic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates surface acoustic wave devices used forresonators, bandpass filters, and other such devices, and moreparticularly, relates to a surface acoustic wave device using arotation-Y plate X propagation LiTaO₃ substrate and a method formanufacturing the same.

2. Description of the Related Art

In mobile communication apparatuses such as mobile phones, surfaceacoustic wave filters have been used, for example, as bandpass filtersor duplexers in the RF stage. As this type of surface acoustic wavefilter, surface acoustic wave filters using a leakage surface acousticwave, which filters are each formed of an IDT (interdigital transducer)made of Al and provided on a 36° to 46° rotation-Y plate X propagationLiTaO₃ substrate, have been used.

However, this surface acoustic wave filter has a poorfrequency-temperature characteristic of −30 ppm/° C. to −40 ppm/° C.,and hence it is necessary to improve the frequency-temperaturecharacteristic. Accordingly, in order to improve thefrequency-temperature characteristic, a resonator has been proposedhaving the structure in which after an IDT composed of Al having anormalized thickness of 0.01 to 0.04 is formed on a 36° rotation-Y plateX propagation (Euler angles: 0°, 126°, 0°) LiTaO₃ substrate, a SiO₂ filmis further provided thereon (for example, in patent publication 1discussed below). In this structure, by forming the SiO₂ film, thefrequency-temperature characteristic is improved.

However, when IDTs are formed by using Al so as to produce filters, inorder to obtain a sufficiently high reflection coefficient orelectromechanical coefficient K_(saw), the IDT must have a relativelylarge electrode thickness H/λ (H indicates the thickness, and λindicates the wavelength of a surface acoustic wave) of 0.08 to 0.10(for example, in literature 2 discussed below).

Patent Publication 1 to be discussed herein refers to JapaneseUnexamined Patent Application Publication No. 1990-295212 and Literature2 to be discussed herein refers to O. Kawachi et al. “Optimum Cut ofLiTaO₃ for High Performance Leaky Surface Acoustic Wave Filters” Proc.1996 IEEE Ultrasonic Symp. pp.71–76 As described above, since the IDTcomposed of Al has a relatively large thickness, when a SiO₂ film isformed at parts shown in FIG. 25( a) for improving thefrequency-temperature characteristic, as shown in FIGS. 25( b) and (c),large steps are formed by the SiO₂ film. As a result, cracks may begenerated therein in some cases. Accordingly, due to the generation ofthe cracks, filter characteristics of the surface acoustic wave filterare degraded.

In addition, since the electrode of the IDT composed of Al is relativelylarge, an effect of covering the irregularities of the electrode surfaceof the IDT cannot be satisfactorily obtained by forming the SiO₂ film,and hence the temperature characteristics may not be sufficientlyimproved.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a surface acoustic wave device using arotation-Y plate X propagation LiTaO₃ substrate and a method formanufacturing the same, in which the frequency-temperaturecharacteristic is greatly improved by the formation of a SiO₂ film andby decreasing the electrode thickness of IDTs, so that cracks areprevented from being generated in the SiO₂ film, the attenuationconstant is also significantly decreased, target electricalcharacteristics such as filter characteristics are achieved, and theelectromechanical coefficient and reflection coefficient of the IDT arealso increased.

According to a preferred embodiment of the present invention, a surfaceacoustic wave device includes a 25° to 55° rotation-Y plate (Eulerangles of (0°, 115° to 145°, 0°)) LiTaO₃ substrate, at least one IDTdisposed on the LiTaO₃ substrate and made of a metal having a densityhigher than that of Al, and a SiO₂ film disposed on the LiTaO₃ substrateso as to cover the IDT.

According to another preferred embodiment of the present invention, asurface acoustic wave device includes a 42° to 50° rotation-Y plate(Euler angles (0°, 132° to 140°, 0°)) LiTaO₃ substrate, at least one IDTwhich is disposed on the LiTaO₃ substrate and is composed of a metalhaving a density higher than that of Al, and a SiO₂ film disposed on theLiTaO₃ substrate so as to cover the IDT.

According to still another preferred embodiment of the presentinvention, a surface acoustic wave device includes a 25° to 42°rotation-Y plate (Euler angles (0°, 115° to 132°, 0°)) LiTaO₃ substrate,at least one IDT which is disposed on the LiTaO₃ substrate and iscomposed of a metal having a density higher than that of Al, and a SiO₂film disposed on the LiTaO₃ substrate so as to cover the IDT.

In preferred embodiments of the present invention, the IDT is preferablymade of at least one metal as a primary component selected from thegroup consisting of Au, Pt, W, Ta, Ag, Mo, Cu, Ni, Co, Cr, Fe, Mn, Zn,and Ti. By using these metals each having a density higher than that ofAl, compared to the case of using Al, the electromechanical coefficientand reflection coefficient of the IDT can be increased (see FIGS. 2 and3 shown below).

According to a more specific preferred embodiment of the presentinvention, the IDT that is preferably is composed of Au and has athickness normalized by the wavelength of an acoustic surface wave inthe range of about 0.013 to about 0.030, and the thickness of the SiO₂film, which is normalized by the wavelength of the acoustic surfacewave, is set in the range of about 0.03 to about 0.45. In this case,according to this preferred embodiment of the present invention, asurface acoustic wave device can be reliably provided in which theelectromechanical coefficient and reflection coefficient are high, thefrequency-temperature characteristic is superior, the attenuationconstant is sufficiently small, and cracking is prevented from occurringin the SiO₂ film.

According to still another preferred embodiment of the presentinvention, the IDT electrode described above is preferably composed ofAu or an Au alloy, and the cut angle of the LiTaO₃ substrate, thenormalized electrode thickness of the IDT, and the normalized thicknessof SiO₂ are shown by one of the following combinations (a) to (k).

TABLE 3 Cut Angle θ Thickness of Au Thickness of SiO₂ (a) 30.0° ≦ θ <33.0° 0.013 to 0.018 0.15 to 0.45 (b) 33.0° ≦ θ < 34.5° 0.013 to 0.0220.10 to 0.40 (c) 34.5° ≦ θ < 35.5° 0.013 to 0.025 0.07 to 0.40 (d) 35.5°≦ θ < 37.5° 0.013 to 0.025 0.06 to 0.40 (e) 37.5° ≦ θ < 39.0° 0.013 to0.028 0.04 to 0.40 (f) 39.0° ≦ θ < 40.0° 0.017 to 0.030 0.03 to 0.42 →preferably 0.022 to 0.028 0.04 to 0.40 (g) 40.0° ≦ θ < 41.5° 0.017 to0.030 0.03 to 0.42 → preferably 0.022 to 0.028 0.04 to 0.40 (h) 41.5° ≦θ < 43.0° 0.018 to 0.028 0.05 to 0.33 (i) 43.0° ≦ θ < 45.0° 0.018 to0.030 0.05 to 0.30 (j) 45.0° ≦ θ ≦ 47.0° 0.019 to 0.032 0.05 to 0.25 (k)47.0° ≦ θ ≦ 50.0° 0.019 to 0.032 0.05 to 0.25

According to still another specific preferred embodiment of the presentinvention, the IDT is composed of Au or an Au alloy, and the cut angleof the LiTaO₃ substrate, the normalized electrode thickness of the IDT,and the normalized thickness of SiO₂ are shown by one of the followingcombinations (m) to (r).

TABLE 4 Cut Angle θ Thickness of Au Thickness of SiO₂ (m) 39.0° ≦ θ <40.0° 0.022 to 0.028 0.04 to 0.40 (n) 40.0° ≦ θ < 41.5° 0.022 to 0.0280.04 to 0.40 (o) 41.5° ≦ θ < 43.0° 0.022 to 0.028 0.05 to 0.33 (p) 43.0°≦ θ < 45.0° 0.022 to 0.030 0.05 to 0.30 (q) 45 0° ≦ θ < 47.0° 0.022 to0.032 0.05 to 0.25 (r) 47.0° ≦ θ ≦ 50.0° 0.022 to 0.032 0.05 to 0.25

According to still another specific preferred embodiment of the surfaceacoustic wave device of the present invention, an adhesive layer isformed between the upper surface of the IDT and the SiO₂ film, and thislayer prevents film peeling of the SiO₂ film from occurring. In thiscase, in addition to the upper surface of the IDT, the adhesive layermay also be formed at the interface between the LiTaO₃ substrate and theSiO₂ film. Furthermore, the adhesive layer may be formed at anapproximately entire interface between the IDT and the SiO₂ film inaddition to the upper surface of the IDT. That is, the adhesive layermay be formed on side surfaces of the IDT.

According to still another specific preferred embodiment of the surfaceacoustic wave device of the present invention, a plurality ofelectrodes, which includes at least electrode pads used for electricalconnection with a bus bar and external elements, are also provided onthe LiTaO₃ substrate. In each of the plurality of electrodes, anunderlying metal layer, which is formed of an underlying electrode layercomposed of a metal having a density higher than that of Al and an uppermetal layer which is formed on the underlying electrode layer and ispreferably composed of Al or an Al alloy, can be formed in the same stepas that for the IDT. In addition, since the upper metal layer iscomposed of Al or an Al alloy, the adhesive strength of the SiO₂ filmcan be increased, and in addition, the cost of the electrodes can bereduced. Furthermore, a wedge-bonding property by Al can also beimproved.

In the surface acoustic wave device of preferred embodiments of thepresent invention, as the surface acoustic wave, a leakage surfaceacoustic wave is preferably used. According to preferred embodiments ofthe present invention, a surface acoustic wave device, which includes anIDT provided with a superior frequency-temperature characteristic, ahigh electromechanical coefficient, and a high reflection coefficient,and which uses a leakage surface acoustic wave having a smallpropagation constant, can be provided.

A method for manufacturing a surface acoustic wave device, according toanother preferred embodiment of the present invention, includes thesteps of preparing a 25° to 55° rotation-Y plate (Euler angles (0°, 115°to 145°, 0°)) LiTaO₃ substrate, forming at least one IDT on the LiTaO₃substrate using a metal having a density higher than that of Al,performing frequency adjustment after the IDT is formed, and forming aSiO₂ film on the LiTaO₃ substrate so as to cover the IDT after thefrequency adjustment is performed.

According to one specific preferred embodiment of the manufacturingmethod of the present invention, as a material for forming the IDT, Auor an alloy primarily composed of Au is preferably used. Since Au has adensity higher than that of Al, an IDT having a high electromechanicalcoefficient and reflection coefficient can be easily formed. Inaddition, the electrode thickness of the IDT can be decreased and cracksgenerated in the SiO₂ film are prevented from occurring. Furthermore,the attenuation constant can be decreased by the presence of the SiO₂film.

The above and other features, elements, characteristics and advantagesof the present invention will be clear from the following detaileddescription of the preferred embodiments of the invention in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a surface acoustic wave device ofone example of preferred embodiments of the present invention.

FIG. 2 is a view showing the relationship between the electromechanicalcoefficient and the normalized electrode thickness of IDTs, the IDTsbeing composed of Au, Ta, Ag, Cr, W, Cu, Zn, Mo, Ni, and Al and beingformed on a 36° rotation-Y plate X propagation (Euler angles (0°, 126°,0°)) LiTaO₃ substrate.

FIG. 3 is a view showing the relationship between the thickness and thereflection coefficient of one side finger electrode of IDTs, the IDTsbeing composed of various electrode materials and being formed on a 36°rotation-Y plate X propagation (Euler angles (0°, 126°, 0°)) LiTaO₃substrate.

FIG. 4 is a view showing the relationship between the normalizedelectrode thickness and the attenuation constant of IDTs, the IDTs beingcomposed of Au, Ta, Ag, Cr, W, Cu, Zn, Mo, Ni, and Al and being formedon a 36° rotation-Y plate X propagation (Euler angles (0°, 126°, 0°))LiTaO₃ substrate.

FIG. 5 is a view showing the change in frequency-temperaturecharacteristic (TCF) when an IDT composed of Au and having a normalizedthickness of 0.02 is formed on a 36° rotation-Y plate X propagation(Euler angles (0°, 126°, 0°)) LiTaO₃ substrate, and SiO₂ films havingvarious thicknesses are formed thereon.

FIG. 6 is a view showing the change in attenuation constant α when IDTscomposed of Au and having various thicknesses are each formed on a 36°rotation-Y plate X propagation (Euler angles (0°, 126°, 0°)) LiTaO₃substrate, and SiO₂ films having various normalized thicknesses areformed thereon.

FIG. 7 is a view showing the change in attenuation constant α when IDTscomposed of Au and having various thicknesses are each formed on a 38°rotation-Y plate X propagation (Euler angles (0°, 128°, 0°)) LiTaO₃substrate, and SiO₂ films having various normalized thicknesses areformed thereon.

FIG. 8 is a view showing attenuation-frequency characteristics of asurface acoustic wave device according to one example and, for the sakeof comparison, those of a surface acoustic wave device before a SiO₂film is formed.

FIGS. 9( a) and (b) are views of scanning electron microscopic picturesshowing the surface states of areas, where an IDT of a surface acousticwave device according to one example is formed, before (a) and after (b)a SiO₂ film is formed thereon.

FIG. 10 is a view showing the change in acoustic velocity of a leakagesurface acoustic wave when the normalized thickness of an IDT composedof Au is variously changed in the structure in which the IDT composed ofAu is formed on a 36° rotation-Y plate X propagation (Euler angles (0°,126°, 0°)) LiTaO₃ substrate, and SiO₂ films having various thicknessesare formed thereon.

FIG. 11 is a view showing the change in acoustic velocity of a leakagesurface acoustic wave when the normalized thickness of a SiO₂ film isvariously changed in the structure in which IDTs composed of Au havingvarious normalized thicknesses are each formed on a 36° rotation-Y plateX propagation (Euler angles (0°, 126°, 0°)) LiTaO₃ substrate, and theSiO₂ film is formed thereon.

FIG. 12 is a view showing the change in electromechanical coefficientwhen the normalized thickness of an IDT composed of Au and having a cutangle θ (Euler angles (0°, θ+90°, 0°)) and the normalized thickness of aSiO₂ film are changed.

FIG. 13 is a view showing the change in Q value when a cut angle θ of aLiTaO₃ substrate and the normalized thickness of a SiO₂ film arechanged.

FIG. 14( a) to (c) are schematic cross-sectional views for illustratingsurface acoustic wave devices each provided with an adhesive layeraccording to modified examples of the present invention.

FIG. 15 is a view showing the relationship between θ and the attenuationconstant α of various Au electrode thicknesses when a SiO₂ film has athickness H/λ of about 0.1.

FIG. 16 is a view showing the relationship between θ and the attenuationconstant α of various Au electrode thicknesses when a SiO₂ film has athickness H/λ of about 0.15.

FIG. 17 is a view showing the relationship between θ and the attenuationconstant α of various Au electrode thicknesses when a SiO₂ film has athickness H/λ of about 0.2.

FIG. 18 is a view showing the relationship between θ and the attenuationconstant α of various Au electrode thicknesses when a SiO₂ film has athickness H/λ of about 0.25.

FIG. 19 is a view showing the relationship between θ and the attenuationconstant α of various Au electrode thicknesses when a SiO₂ film has athickness H/λ of about 0.3.

FIG. 20 is a view showing the relationship between θ and the attenuationconstant α of various Au electrode thicknesses when a SiO₂ film has athickness H/λ of about 0.35.

FIG. 21 is a view showing the relationship between θ and the attenuationconstant α of various Au electrode thicknesses when a SiO₂ film has athickness H/λ of about 0.4.

FIG. 22 is a view showing the relationship between θ and the attenuationconstant α of various Au electrode thicknesses when a SiO₂ film has athickness H/λ of about 0.45.

FIG. 23 is a view showing an equivalent circuit of a surface acousticwave resonator according to one example of preferred embodiments of thepresent invention.

FIG. 24 is a view showing the relationship between the normalizedthickness of a SiO₂ film in a surface acoustic wave resonator accordingto preferred embodiments of the present invention and the equivalentseries resistance when the surface acoustic wave resonator is fitted ina resonant circuit.

FIGS. 25( a), (b), and (c) are scanning electron microscopic picturesfor illustrating problems of a conventional surface acoustic wavedevice, (a) shows a state before a SiO₂ film is formed, and (b) shows astate of a surface and a cross-section of the SiO₂ film after itsformation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, with reference to the figures, the present invention willbe described in conjunction with particular examples of preferredembodiment of the present invention.

FIG. 1 is a plan view for illustrating a longitudinally coupled resonantfilter as a surface acoustic wave device according to one example ofpreferred embodiments of the present invention.

A surface acoustic wave device 11 has the structure in which IDTs 13 aand 13 b and reflectors 14 a and 14 b are disposed on the upper surfaceof a LiTaO₃ substrate 12. In addition, a SiO₂ film 15 is arranged so asto cover the IDTs 13 a and 13 b and the reflectors 14 a and 14 b.Related to this, as the LiTaO₃ substrate 12, a 25° to 55° rotation-Yplate X propagation (Euler angles (0°, 115° to 145°, 0)) LiTaO₃substrate is preferably used. In a rotation-Y plate X propagation LiTaO₃substrate having a cut angle outside the range described above, theattenuation constant is high, and TCF is also degraded.

The IDTs 13 a and 13 b and the reflectors 14 a and 14 b are preferablymade of a metal having a density higher than that of Al. As the metalmentioned above, for example, there may be at least one metal selectedfrom the group consisting of Au, Pt, W, Ta, Ag, Mo, Cu, Ni, Co, Cr, Fe,Mn, Zn, and Ti, or an alloy primarily composed of at least one of themetals mentioned above.

As described above, since the IDTs 13 a and 13 b and the reflectors 14 aand 14 b are preferably made of a metal having a density higher thanthat of Al, even when the thicknesses are small as compared to the casein which the IDTs 13 a and 13 b and the reflectors 14 a and 14 b arecomposed of Al, as shown in FIGS. 2 and 3, the electromechanicalcoefficient and reflection coefficient can be increased.

In addition, since the electrode thickness can be decreased as describedabove, generation of cracks caused by the steps of the SiO₂ film 15formed on the IDTs 13 a and 13 b is reliably prevented. Concerning thethickness of the SiO₂ film 15, as it will be apparent from theexperimental examples described later, the thickness H/λ, which isnormalized by the wavelength of a surface acoustic wave, is preferablyin the range of about 0.03 to about 0.45, where H indicates thethickness, and λ indicates the wavelength of the surface acoustic wave.By being set in the range described above, the attenuation constant canbe significantly decreased compared to the case in which the SiO₂ filmis not provided, and hence low loss can be achieved.

Although depending on a material for forming the IDT, for example, whenan Au film is used, the film thicknesses of the IDTs 13 a and 13 b,normalized by the wavelength of the surface acoustic wave, arepreferably in the range of about 0.013 to about 0.030. When the Au filmis thin, since the IDT has lead-wire resistance, the film thickness ismore preferably in the range of about 0.021 to about 0.03.

In the surface acoustic wave device of preferred embodiments of thepresent invention, as described above, the IDTs 13 a and 13 b arepreferably made of a metal having a density higher than that of Al, andthe electrode thicknesses of the IDTs 13 a and 13 b can be decreased.Accordingly, the generation of the steps of the SiO₂ film does notoccur, and hence cracking can be reliably prevented. In addition, theattenuation constant can be significantly decreased by the SiO₂ film,and hence low loss can be achieved. Hence, superior properties can beobtained, and by the formation of the SiO₂ film 15, a preferablefrequency-temperature characteristic can be realized. Hereinafter, theadvantages described above will be described with reference toparticular examples.

The changes in electromechanical coefficient K_(saw), attenuationconstant (α), and reflection coefficient |ref| are shown in FIGS. 2, 3,and 4, respectively, in the case in which an IDT composed of Al and IDTscomposed of Au, Ta, Ag, Cr, W, Cu, Zn, Mo, and Ni are formed on a 36°rotation-Y plate X propagation LiTaO₃ substrate so as to have variousthicknesses. Related to this, calculation of numerical values wasperformed in accordance with the method described in J. J. Campbell andW. R. Jones: IEEE Trans. Sonic & Ultrasonic. SU-15. p 209 (1968), andthe calculation was performed on the assumption that the electrodes areuniform on the entire surface.

As can be seen from FIG. 2, in the IDT composed of Al, when thenormalized thickness H/λ is about 0.10, the electromechanicalcoefficient is approximately 0.27. In the case described above, Hindicates the thickness, and λ indicates the wavelength of a surfaceacoustic wave. On the contrary, when the IDTs composed of Au, Ta, Ag,Cr, W, Cu, Zn, Mo, and Ni have the H/λ set in the range of from about0.013 to about 0.035, an even higher electromechanical coefficientK_(saw) can be realized. However, as can be seen in FIG. 4, regardlessof the film thickness H/λ, compared to the case in which the IDTcomposed of Al has an attenuation constant (α) of approximately zero,the attenuation constants of the IDTs composed of Au, Ta, Ag, Cr, W, Cu,Zn, Mo, and Ni become very high.

In addition, FIG. 12 is a view showing the relationship between theelectromechanical coefficient and θ in the structure in which an IDTcomposed of Au and a SiO₂ film are formed on a LiTaO₃ substrate having acut angle θ (Euler angles (0°, θ+90°, 0°). In this case, the normalizedthickness of the IDT composed of Au was set to approximately 0.022,0.025, and 0.030, and the SiO₂ films were formed having a normalizedthickness of 0.00 (no SiO₂ film formation), and approximately 0.10,0.20, 0.30, and 0.45.

As can be seen in FIG. 12, it is understood that the electromechanicalcoefficient K_(saw) is decreased concomitant with the increase inthickness of the SiO₂ film. In addition, as described later, in order tosuppress the properties degradation caused by the SiO₂ film, the case inwhich the film thickness of the IDT is decreased is considered. As canbe seen in FIG. 2 described above, when the normalized thickness of aconventional IDT composed of Al is decreased to about 0.04, although theSiO₂ film is not formed, the electromechanical coefficient K_(saw) isdecreased to about 0.245. In addition, when the normalized thickness ofthe IDT composed of Al is set to about 0.04, and the SiO₂ film isformed, the electromechanical coefficient K_(saw) is further decreased,and hence broad-band characteristics cannot be practically obtained.

On the contrary, as can be seen in FIG. 12, in the structure in whichthe IDT is composed of Au, and the SiO₂ film is formed, when the cutangle is set to about 38.5° or less, it is found that theelectromechanical coefficient K_(saw) is about 0.245 or more althoughthe normalized thickness of the SiO₂ film is set to approximately 0.45.In addition, when a SiO₂ film having a normalized thickness ofapproximately 0.30 is formed, by setting the cut angle θ to 42° or less,S electromechanical coefficient K_(saw) can be about 0.245 or more.Related to this, as described later, when the cut angle is less than25°, the attenuation constant is increased and hence cannot bepractically used. Accordingly, it is understood that a 25° to 42°rotation-Y plate X propagation LiTaO₃ substrate (Euler angles (0°, 115°to 132°, 0°) is preferably used, and a 25° to 38.50 rotation-Y plate Xpropagation LiTaO₃ substrate (Euler angles (0°, 115° to 128.5°, 0°) ismore preferably used.

On the other hand, the frequency-temperature characteristic (TCF) of a36° rotation-Y plate X propagation LiTaO₃ substrate was −30 ppm/° C. to−40 ppm/° C. and is not satisfactory. In order to improve thisfrequency-temperature characteristic, on a 36° rotation-Y plate Xpropagation LiTaO₃ substrate, an IDT composed of Au is formed, and SiO₂films having various thicknesses are further formed. The changes infrequency-temperature characteristic thereof are shown in FIG. 5. InFIG. 5, O indicates a theoretical value, and x indicates an experimentalvalue. In this example, the normalized thickness H/λ of the IDT composedof Au is about 0.020.

As can be seen in FIG. 5, it is understood that thefrequency-temperature characteristic can be improved by the formation ofthe SiO₂ film. In particular, when the normalized thickness H/λ of theSiO₂ film is approximately 0.25, it is understood that the TCFpreferably becomes zero.

In addition, the numerical analysis of the change in attenuationconstant α was carried out by variously changing the film thicknesses ofthe IDT composed of Au and the SiO₂ film using two types of LiTaO₃substrates having a cut angle of 36° (Euler angles (0°, 126°, 0°)) and acut angle of 38° (Euler angles (0°, 128°, 0°)) as the rotation-Y plate Xpropagation LiTaO₃ substrate. The film thickness of Au shown in FIGS. 6and 7 is represented by H/λ. The results are shown in FIGS. 6 and 7. Ascan be seen from FIGS. 6 and 7, regardless of the film thickness of theIDT composed of Au, it is understood that the attenuation constant α canbe decreased when the thickness of the SiO₂ film is selected. That is,as can be seen from FIGS. 6 and 7, as long as the film thickness H/λ ofthe SiO₂ film is set to about 0.03 to about 0.45, and more preferably,is set in the range of from about 0.10 to about 0.35, it is understoodthat the attenuation constant α can be significantly decreased wheneveran IDT composed of Au having any thickness is provided on either of theLiTaO₃ substrates having the cut angles described above.

In addition, as shown in FIG. 3, when the IDT composed of Au is used, itis understood that a sufficiently high reflection coefficient can beobtained even though the film thickness is small as compared to thatcomposed of Al.

Accordingly, from the results shown in FIGS. 2 to 7, in the case inwhich the IDT composed of Au having a film thickness H/λ of about 0.013to about 0.030 is formed on the LiTaO₃ substrate, when the filmthickness H/λ of the SiO₂ film is set in the range of about 0.03 toabout 0.45, in addition to a high electromechanical coefficient, asignificantly small attenuation constant α can be obtained, and asufficient reflection coefficient can be obtained.

In the example described above, a surface acoustic wave device 11 wasformed having the structure in which on a LiTaO₃ substrate having a cutangle of 36° (Euler angles (0°, 126°, 0°)), an IDT composed of Au havinga normalized thickness H/λ of about 0.020 was formed, and a SiO₂ filmhaving a normalized thickness H/λ of about 0.1 was then formed thereon,and attenuation-frequency characteristics of the surface acoustic wavedevice 11 are shown by dashed lines in FIG. 8. In addition, for the sakeof comparison, the attenuation-frequency characteristics of this surfaceacoustic wave device before the SiO₂ film is formed are shown by thesolid lines.

As can be seen in FIG. 8, it is understood that although theelectromechanical coefficient is slightly decreased from about 0.30 toabout 0.28 by the formation of the SiO₂ film, the insertion loss isimproved. Accordingly, as can be seen in FIG. 8, it is confirmed thatthe attenuation constant α is decreased when the SiO₂ film is formed tohave the specific thickness range described above.

In addition, FIGS. 9( a) and (b) are scanning electron microscopicpictures showing the surfaces of the surface acoustic wave deviceaccording to the above-described example. In these pictures, the statesbefore and after a SiO₂ film having a normalized thickness H/λ of about0.3 is formed on an IDT composed of Au having a normalized thickness H/λof about 0.02 are shown. As can be seen in FIG. 9( b) showing the stateafter the film is formed, no cracks are observed on the surface of theSiO₂ film, and hence it is understood that degradation of propertiescaused by cracks is prevented from occurring.

The inventors of the present invention formed IDTs composed of Au havinga normalized thickness of about 0.02 on rotation-Y plate X propagationLiTaO₃ substrates having various cut angles and then further formed SiO₂films having various thicknesses, thereby forming experimental one porttype surface acoustic wave resonators. In the case described above, thenormalized thicknesses of the SiO₂ film were set to approximately 0.10,0.20, 0.30, and 0.45. The Q values of the one port type surface acousticwave resonators thus formed were measured. The results are shown in FIG.13.

In general, when the Q value of a resonator is increased, the steepnessof filter characteristics is greatly improved from the pass band to theattenuation band when the resonator is used as a filter. Accordingly,when a filter having steep characteristics is required, a higher Q valueis preferable. As can be seen in FIG. 13, it is understood thatregardless of the film thickness of the SiO₂ film, the Q value becomesmaximum at a cut angle of approximately 48° rotation-Y plate and isrelatively large at a cut angle in the range of about 42° to about 58°.

Accordingly, as can be seen in FIG. 13, when the structure includes aLiTaO₃ substrate having a cut angle of about 42° to about 58° rotation-Yplate (Euler angles (0°, 132° to 148°, 0°)), and at least one IDTcomposed of a metal having a density higher than that of Au and a SiO₂film are formed on the LiTaO₃ substrate so as to cover the IDT, it isunderstood that a high Q value can be obtained. As can be seen in FIG.13, the cut angle is preferably about 46.5° to about 53° rotation-Yplate (Euler angles (0°, 136.5° to 143°, 0°))

In preferred embodiments of the present invention, an adhesive layer maybe provided on the upper surface of the IDT. That is, as shown in FIG.14( a), on a LiTaO₃ substrate 22, an IDT 23 is formed, and on the uppersurface of the IDT 23, an adhesive layer 24 may be formed. The adhesivelayer 24 is disposed between the IDT 23 and a SiO₂ film 25. The adhesivelayer 24 is provided to increase an adhesive strength of the SiO₂ film25 to the IDT 23. As a material for forming the adhesive layer 24described above, Pd, Al, or an alloy thereof is preferably used. Inaddition, besides the metals, a piezoelectric material such as ZnO oranother ceramic such as Ta₂O₃ or Al₂O₃ may be used for forming theadhesive layer 24. By the formation of the adhesive layer 24, theadhesive strength between the IDT 23 formed of a metal having a densityhigher than that of Al and the SiO₂ film 25 is increased, and hencepeeling of the SiO₂ film is reliably prevented.

The thickness of the adhesive layer 24 is preferably set toapproximately one percent of the wavelength of the surface acoustic waveso as not to influence the entirety of the surface acoustic wave. Inaddition, in FIG. 14( a), the adhesive layer 24 is formed on the uppersurface of the IDT 23. However, as shown in FIG. 14( b), an adhesivelayer 24A may be formed at the interface of the SiO₂ film 25 on theLiTaO₃ substrate. Furthermore, as shown in FIG. 14( c), the adhesivelayer 24 may be formed on side surfaces of the IDT 23 in addition to theupper surface thereof.

In addition, as another structure for improving the adhesive strength ofthe SiO₂ film, a plurality of electrodes including electrode pads usedfor electrode connection with a bus bar and the outside, other than theIDT, may be formed of an underlying metal layer composed of the samematerial as that for the IDT and an upper metal layer which is providedon the underlying metal layer and is composed of Al or an Al alloy. Thatis, for example, as electrode films forming the reflectors 14 a and 14 bshown in FIG. 1, a film may be formed containing an Al film provided onan underlying metal layer composed of the same material as that for theIDTs 13 a and 13 b. As described above, by providing the upper metallayer composed of Al or an Al alloy, the adhesion strength to the SiO₂film is greatly increased. In addition, the electrode cost can bereduced, and an Al wedge-bonding property can also be improved.

As the electrodes other than the IDT, in addition to the pads used forelectrode connection with the bus bar and the outside, lead electrodesprovided whenever necessary may also be mentioned. In addition, the Alalloy is not specifically limited. However, an Al—Ti alloy or Al—Ni—Cralloy may be mentioned by way of example.

In the case in which a rotation-Y plate X propagation LiTaO₃ substratehaving a cut angle different from that described in the above example,when an IDT composed of Au is formed, it was also confirmed by theinventors of preferred embodiments of the present invention that theattenuation constant α can be minimized by a SiO₂ film having a specificfilm thickness. That is, when the film thickness of the SiO₂ film is setin a specific range, as in the case of the above example, theattenuation constant α can be decreased. In addition, the relationshipbetween the cut angles and α is shown in FIGS. 15 to 22 when the filmthickness H/λ of the SiO₂ film is set in the range of about 0.1 to about0.45. From these figures, it was found that, with increase in SiO₂ filmthickness, the cut angle at which a is minimized becomes smaller.Accordingly, even when a rotation-Y plate X propagation LiTaO₃ substratehaving a cut angle different from that mentioned above is used, as longas an IDT composed of Au is formed, a SiO₂ film is provided thereon, andthe film thickness of the SiO₂ film is selected, a surface acoustic wavedevice can be formed having a high electromechanical coefficient, a highreflection coefficient, and a superior frequency-temperaturecharacteristic TCF, which is approximately less than half compared to aconventional surface acoustic wave device. It was confirmed thatpreferable combinations of the cut angle of the LiTaO₃ substrate, theelectrode thickness of the IDT composed of Au, and the film thickness ofthe SiO₂ film, which can realize the effects described above, arerepresented by the following combinations from (a) to (k) and (m) to(r). However, due to the variation in metallization ratio, materialconstant, and other factors, it has been considered that the rotationcut angle may be deviated by approximately ±40 from the value describedabove.

TABLE 5 Cut Angle θ Thickness of Au Thickness of SiO₂ (a) 30.0° ≦ θ <33.0° 0.013 to 0.018 0.15 to 0.45 (b) 33.0° ≦ θ < 34.5° 0.013 to 0.0220.10 to 0.40 (c) 34.5° ≦ θ < 35.5° 0.013 to 0.025 0.07 to 0.40 (d) 35.5°≦ θ < 37.5° 0.013 to 0.025 0.06 to 0.40 (e) 37.5° ≦ θ < 39.0° 0.013 to0.028 0.04 to 0.40 (f) 39.0° ≦ θ < 40.0° 0.017 to 0.030 0.03 to 0.42 →preferably 0.022 to 0.028 0.04 to 0.40 (g) 40.0° ≦ θ < 41.5° 0.017 to0.030 0.03 to 0.42 → preferably 0.022 to 0.028 0.04 to 0.40 (h) 41.5° ≦θ < 43.0° 0.018 to 0.028 0.05 to 0.33 (i) 43.0° ≦ θ < 45.0° 0.018 to0.030 0.05 to 0.30 (j) 45.0° ≦ θ ≦ 47.0° 0.019 to 0.032 0.05 to 0.25 (k)47.0° ≦ θ ≦ 50.0° 0.019 to 0.032 0.05 to 0.25

TABLE 6 Cut Angle θ Thickness of Au Thickness of SiO₂ (m) 39.0° ≦ θ <40.0° 0.022 to 0.028 0.04 to 0.40 (n) 40.0° ≦ θ < 41.5° 0.022 to 0.0280.04 to 0.40 (o) 41.5° ≦ θ < 43.0° 0.022 to 0.028 0.05 to 0.33 (p) 43.0°≦ θ < 45.0° 0.022 to 0.030 0.05 to 0.30 (q) 45.0° ≦ θ < 47.0° 0.022 to0.032 0.05 to 0.25 (r) 47.0° ≦ θ ≦ 50.0° 0.022 to 0.032 0.05 to 0.25

FIG. 24 shows equivalent series resistances R1 of resonant circuitsshown in FIG. 23 fitted with surface acoustic wave resonators of 900 MHzband, which are produced by forming IDTs each composed of Au and havinga normalized thickness of about 0.02 on a 36° rotation-Y plate Xpropagation (Euler angles (0°, 126°, 0°)) LiTaO₃ substrate, and formingSiO₂ films having various thicknesses thereon. Related to this, theequivalent series resistance R1 of the resonant circuit fitted with thesurface acoustic wave resonator represents approximately loss caused byelectrode resistance and loss caused by attenuation of the surfaceacoustic wave. Accordingly, when the resistance of the electrode isapproximately constant, the tendency of R1 approximately coincides withthat of α (attenuation constant).

As can be seen in FIG. 24, compared to the case in which the SiO₂ filmis not provided, it is found that when the SiO₂ film is formed, R1 isdecreased, and when the normalized thickness of the SiO₂ film is about0.02 or more, the equivalent series resistance R1 is decreased. Thiscoincides with the tendency shown in FIG. 6.

When the surface acoustic wave device of preferred embodiments of thepresent invention is manufactured, it is preferable that frequencyadjustment be performed in the state in which an IDT primarily composedof Au is formed on a rotation-Y plate X propagation LiTaO₃ substrate,and that a SiO₂ film be then formed having a thickness in the range inwhich the attenuation constant α can be decreased. These steps will bedescribed with reference to FIGS. 10 and 11. FIG. 10 shows the change inacoustic velocity of a leakage surface acoustic wave in the case inwhich IDTs composed of Au having various thicknesses and SiO₂ filmsprovided thereon and having various thicknesses are formed on a 36°rotation-Y plate X propagation (Euler angles (0°, 126°, 0°)) LiTaO₃substrate. In addition, FIG. 11 shows the change in acoustic velocity ofa leakage surface acoustic wave in the case in which IDTs composed of Auhaving various thicknesses are formed on a LiTaO₃ substrate having thesame Euler angles as described above, and SiO₂ films having variousnormalized thicknesses are formed on the IDTs. As it is clearlyunderstood when FIGS. 10 and 11 are compared to each other that thechange in acoustic velocity of the surface acoustic wave issignificantly large when the thickness of Au is changed as compared tothe case in which the thickness of the SiO₂ film is changed.Accordingly, prior to the formation of the SiO₂ film, frequencyadjustment is preferably performed, and after the IDT composed of Au isformed, for example, by laser etching or ion etching, frequencyadjustment is preferably performed. When the normalized thickness of Auis in the range of from about 0.015 to about 0.03, it is particularlypreferable since the change in acoustic velocity caused by the SiO₂ filmis decreased, and hence variation in frequency caused by variation ofthe SiO₂ film can be decreased.

In the experimental examples described above, as a metal forming theIDT, Au is described by way of example. However, it has been confirmedby the inventors of the present invention that when Pt, W, Ta, Ag, Mo,Cu, Ni, Co, Cr, Fe, Mn, Zn, or Ti is used, and the thickness of the SiO₂film is selected as described above, the electromechanical coefficientand reflection coefficient are significantly increased, thefrequency-temperature characteristic is improved, and cracking of theSiO₂ film can be prevented.

In addition, in order to improve adhesion strength of the electrodes, avery thin Ti or Cr film may be formed under the Au or Ag electrodes.

In addition to the longitudinally coupled resonator type surfaceacoustic wave filter shown in FIG. 1, the present invention can beapplied to various surface acoustic wave devices, such as surfaceacoustic wave resonators, laterally coupled type surface acoustic wavefilters, ladder-type filters, and lattice-type filters.

In the surface acoustic wave device of various preferred embodiments ofthe present invention, at least one IDT composed of a metal having adensity higher than that of Al is formed on a 25° to 55° rotation-Yplate X propagation LiTaO₃ substrate, and a SiO₂ film is formed so as tocover the IDT. Accordingly, the frequency-temperature characteristic canbe improved by the formation of the SiO₂ film. In addition, compared tothe case in which Al is used, the electrode thickness of the IDT can bedecreased, the generation of cracks in the SiO₂ film can be prevented,and target properties can be reliably obtained. Furthermore, when theIDT is composed of a metal having a density higher than that of Al, theattenuation constant α may be degraded in some cases. However, by theformation of the SiO₂ film, the degradation of the attenuation constanta is reliably prevented.

Hence, according to preferred embodiments of the present invention, inaddition to the improvement in frequency-temperature characteristicperformed by the SiO₂ film, the generation of cracks in the SiO₂ filmcan also be prevented, and as a result, desired properties can bereliably obtained.

In addition, according to the method for manufacturing a surfaceacoustic wave device of preferred embodiments of the present invention,the surface acoustic wave device of preferred embodiments of the presentinvention can be formed having a superior frequency-temperaturecharacteristic as described above, in which the generation of cracks inthe SiO₂ film is prevented, and desired properties can be reliablyobtained. In addition to these advantages described above, sincefrequency adjustment is performed after the formation of the IDT, andthe SiO₂ film is formed after the frequency adjustment, the frequencyadjustment can be more precisely performed, and hence the effects offrequency variation caused by the variation in thickness of the SiO₂film are minimized. Consequently, a surface acoustic wave device havingdesired frequency characteristics can be reliably provided.

While the present invention has been described with reference to whatare at present considered to be the preferred embodiments, it is to beunderstood that various changes and modifications may be made theretowithout departing from the invention in its broader aspects andtherefore, it is intended that the appended claims cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

1. A surface acoustic wave device comprising: a 25° to 55° rotation-Yplate LiTaO₃ substrate having Euler angles of (0°, 115° to 145°, 0°); atleast one IDT disposed on the LiTaO₃ substrate and made of a metalhaving a density higher than that of Al; an SiO₂ film arranged on theLiTaO₃ substrate so as to cover the at least one IDT; and an adhesivelayer disposed between the upper surface of the IDT and the SiO₂ film.2. A surface acoustic wave device according to claim 1, wherein theadhesive layer is also disposed at an interface between the LiTaO₃substrate and the SiO₂ film.
 3. A surface acoustic wave device accordingto claim 1, wherein the adhesive layer is disposed on an upper surfaceof the IDT and is disposed at the entire interface between the SiO₂ filmand the IDT.
 4. A surface acoustic wave device comprising: a 25° to 55°rotation-Y plate LiTaO₃ substrate having Euler angles of (0°, 115° to145°, 0°); at least one IDT disposed on the LiTaO₃ substrate and made ofa metal having a density higher than that of Al; an SiO₂ film arrangedon the LiTaO₃ substrate so as to cover the at least one IDT; and aplurality of electrodes disposed on the LiTaO₃ substrate and whichincludes at least electrode pads used for electrode connection with abus bar and external elements, wherein each of the electrodes includesan underlying electrode layer made of a metal having a density higherthan that of Al and an upper metal layer which is disposed on theunderlying electrode layer and is made of one of Al and an Al alloy. 5.A surface acoustic wave device comprising: a 42° to 50° rotation-Y plateLiTaO₃ substrate having Euler angles of (0°, 132° to 140°, 0°); at leastone IDT disposed on the LiTaO₃ substrate and made of a metal having adensity higher than that of Al; a SiO₂ film arranged on the LiTaO₃substrate so as to cover the at least one IDT; and an adhesive layerdisposed between the upper surface of the IDT and the SiO₂ film.
 6. Asurface acoustic wave device according to claim 5 wherein the adhesivelayer is also disposed at an interface between the LiTaO₃ substrate andthe SiO₂ film.
 7. A surface acoustic wave device according to claim 5,wherein the adhesive layer is disposed on an upper surface of the IDTand is disposed at the entire interface between the SiO₂ film and theIDT.
 8. A surface acoustic wave device comprising: a 42° to 50°rotation-Y plate LiTaO₃ substrate having Euler angles of (0°, 132° to140°, 0°); at least one IDT disposed on the LiTaO₃ substrate and made ofa metal having a density higher than that of Al; and a SiO₂ filmarranged on the LiTaO₃ substrate so as to cover the at least one IDT;and a plurality of electrodes disposed on the LiTaO₃ substrate and whichincludes at least electrode pads used for electrode connection with abus bar and external elements, wherein each of the electrodes includesan underlying electrode layer made of a metal having a density higherthan that of Al and an upper metal layer which is disposed on theunderlying electrode layer and is made of one of Al and an Al alloy. 9.A surface acoustic wave device comprising: a 25° to 42° rotation-Y plateLiTaO₃ substrate having Euler angles of (0°, 115° to 132°, 0°); at leastone IDT disposed on the LiTaO₃ substrate and made of a metal having adensity higher than that of Al; and a SiO₂ film arranged on the LiTaO₃substrate so as to cover the at least one IDT; an an adhesive layerdisposed between the upper surface of the IDT and the SiO₂ film.
 10. Asurface acoustic wave device according to claim 9, wherein the adhesivelayer is also disposed at an interface between the LiTaO₃ substrate andthe SiO₂ film.
 11. A surface acoustic wave device according to claim 9,wherein the adhesive layer is disposed on an upper surface of the IDTand is disposed at the entire interface between the SiO₂ film and theIDT.
 12. A surface acoustic wave device comprising: a 25° to 42°rotation-Y plate LiTaO₃ substrate having Euler angles of (0°, 115° to132°, 0°); at least one IDT disposed on the LiTaO₃ substrate and made ofa metal having a density higher than that of Al; and a SiO₂ filmarranged on the LiTaO₃ substrate so as to cover the at least one IDT;and a plurality of electrodes disposed on the LiTaO₃ substrate and whichincludes at least electrode pads used for electrode connection with abus bar and external elements, wherein each of the electrodes includesan underlying electrode layer made of a metal having a density higherthan that of Al and an upper metal layer which is disposed on theunderlying electrode layer and is made of one of Al and an Al alloy.